Disposable products such as facial tissue, sanitary tissue, paper towels, and the like are typically made from one or more webs of paper. If the products are to perform their intended tasks, the paper webs from which they are formed must exhibit certain physical characteristics. Among the more important of these characteristics are strength, softness, and absorbency. Strength is the ability of a paper web to retain its physical integrity during use. Softness is the pleasing tactile sensation the user perceives as the user crumples the paper in his or her hand and contacts various portions of his or her anatomy with the paper web. Softness generally increases as the paper web stiffness decreases. Absorbency is the characteristic of the paper web which allows it to take up and retain fluids. Typically, the softness and/or absorbency of a paper web is increased at the expense of the strength of the paper web. Accordingly, papermaking methods have been developed in an attempt to provide soft and absorbent paper webs having desirable strength characteristics.
U.S. Pat. No. 3,301,746 issued to Sanford et al. discloses a paper web which is thermally pre-dried with a through air-drying system. Portions of the web are then impacted with a fabric knuckle pattern at the dryer drum. While the process of Sanford et al. is directed to providing improved softness and absorbency without sacrificing tensile strength, water removal using the through-air dryers of Sanford et al. is very energy intensive, and therefore expensive.
U.S. Pat. No. 3,537,954 issued to Justus discloses a web formed between an upper fabric and a lower forming wire. A pattern is imparted to the web at a nip where the web is sandwiched between the fabric and a relatively soft and resilient papermaking felt. U.S. Pat. No. 4,309,246 issued to Hulit et al. discloses delivering an uncompacted wet web to an open mesh imprinting fabric formed of woven elements, and pressing the web between a papermaker's felt and the imprinting fabric in a first press nip. The web is then carried by the imprinting fabric from the first press nip to a second press nip at a drying drum. U.S. Pat. No. 4,144,124 issued to Turunen et al. discloses a paper machine having a twin-wire former having a pair of endless fabrics, which can be felts. One of the endless fabrics carries a paper web to a press section. The press section can include the endless fabric which carries the paper web to the press section, an additional endless fabric which can be a felt, and a wire for pattern embossing the web.
Both Justus and Hulit et al. suffer from the disadvantage that they press a wet web in a nip having only one felt. During pressing of the web, water will exit both sides of the web. Accordingly, water exiting the surface of the web which is not in contact with a felt can re-enter the web at the exit of the press nip. Such re-wetting of the web at the exit of the press nip reduces the water removal capability of the press arrangement, disrupts fiber-to-fiber bonds formed during pressing, and can result in rebulking of the portions of the web which are densified in the press nip.
Turunen et al. discloses a press nip which includes two endless fabrics, which can be felts, and an imprinting wire. However, Turunen et al. does not transfer the web from a forming wire to an imprinting fabric to provide initial deflection of portions of the wet web into the imprinting fabric prior to pressing the web in the press nip. The web in Turunen can therefore be generally monoplanar at the entrance to the press nip, resulting in overall compaction of the web in the press nip. Overall compaction of the web is undesirable because it limits the difference in density between different portions of the web by increasing the density of relatively low density portions of the web.
In addition, Hulit et al., and Turunen et al. provide press arrangements wherein the imprinting fabric has discrete compaction knuckles, such as at the warp and weft crossover points of woven filaments. Discrete compacted sites do not provide a wet molded sheet having a continuous high density region for carrying loads and discrete low density regions for providing absorbency.
Embossing can also be used to impart bulk to a web. However, embossing of a dried web can result in disruption of bonds between fibers in the web. This disruption occurs because the bonds are formed and then set upon drying of the web. After the web is dried, moving fibers normal to the plane of the web disrupts fiber to fiber bonds, which in turn results in a web having less tensile strength than existed before embossing.
In conventional pressed papermaking operations employing two felts, the paper web is positioned between to two felts. One side of the paper web is in contact with one of the felts, and the other side of the paper web is in contact with the other felt. At the exit of the nip, the paper web follows one of the felts. The other felt is separated from the paper web. It is important that the web follow the intended felt, so that the web is directed to the appropriate downstream operations.
To ensure the web follows the intended felt, conventional pressed papermaking operations use two felts having different structures. The felt which is intended to carry the paper web from the nip has a finer, more dense construction than the felt which is to be separated from the web at the nip exit. The felt having a finer, more dense construction is characterized by having a lower air permeability than the other felt. The finer, more dense construction of the felt carrying the paper web from the nip exit helps ensure that the web follows that felt, thereby avoiding unintentional transfer of the web to the other felt.
Paper scientists continue to search for improved paper structures that can be produced economically, and which provide increased strength without sacrificing softness and absorbency.
One object of the present invention is to provide a method for dewatering and molding a paper web.
Another object of the present invention is to press a web and an imprinting member between two felt layers, wherein one felt, which is in flow communication with conduits in the imprinting member, has a relatively high air permeability, and wherein the other felt, which is positioned adjacent a surface of the web, can have a relatively low air permeability.
Another object of the present invention is to provide a non-embossed patterned paper web having a relatively high density continuous network, and a plurality of relatively low density domes dispersed throughout the continuous network.